Do bear in mind if you decide to download any emulated games (ROMs) that their legality is questionable--there's no real consensus on what is an appropriate usage for them. Generally speaking, most agree that it's best to only download games that you yourself own the physical copies of. Some also add that for games you do not own, there's a 24-hour evaluation period permitted, after which you should delete the ROMs from your system.
In any case, I would be cautious. The original games aren't on store shelves any longer, but Nintendo is definitely still profitting from them (look at the various ports and updated releases on their handheld systems). I'd also take your search to Google if you need to know more, since we don't want this forum to be liable for any potentially illegal activity.
Most ROMs are illegal to download (but I think not to own) as far as I know. I definitely know that a ROM of any product still available in retail is illegal to download, and a lot of products no longer available still have licensing agreements that prohibit ROM sharing. That said, EVERY NES/SNES/N64 ROM is, as far as I know, illegal to download, or definitely will be shortly, as the WII is going to have an online pay service allowing you to buy/download the old games to your WII system at around $5 a pop.